Method for rendering inert dust residue containing silicon metal

ABSTRACT

The invention relates to a method for rendering inert dust residue containing silicon metal, left over from trichlorosilane synthesis. The inventive method produces valuable materials containing silicon which can be used in metallurgical processes. According to said method, the process of rendering the residue inert is carried out in several steps. In a first step, 10 to 50 wt. % water, in relation to the quantity of residue, and an at least equimolar quantity of an alkaline compound, in relation to the chloride content of the residue, are added to the residue. Subsequently or simultaneously, the residue is heated to a temperature of 50 to 200° C. In a further step, at least twice the quantity of water is added to this mixture, which is extensively liberated from dissolved salts by filtration and subsequent washing with water.

[0001] The invention relates to a method for rendering inert dustresidue containing silicon metal originating from the trichlorosilanesynthesis in a fluidised bed as well as the use of such inert residuesin metallurgical processes.

[0002] Trichlorosilane is produced in the technical scope in accordancewith the “Studies in Organic Chemistry 49; Catalyzed Direct Reactions ofSilicon”, Elsevier, 1993, p. 445 to 457 by reacting metallurgicalsilicon with hydrogen chloride or by reacting silicon withtetrachlorosilane and hydrogen. As a rule, such processes are carriedout in fluidised-bed reactors adding catalysts, like e.g. copper orcopper compounds, as well as without adding any catalyst.

[0003] All such processes have in common that thetrichlorosilane-containing gaseous product mixture that leaves thefluidised bed is contaminated by solid dust containing silicon and, ifany, catalyst. The removal of such dust is achieved by separation in acyclone or in several cyclones which are connected in series or byhot-gas filtration with ceramic or sintered metal candles. Any dustresidue remaining in the trichlorosilane-containing gaseous productmixture is subsequently removed by washing with liquid chlorosilanes.

[0004] Alternatively, procedures are known, wherein the dust is removedfrom the trichlorosilane-containing product mixture exclusively bywashing with liquid chlorosilanes or by total condensation of theproduct mixture and subsequent distillation without previous separationby means of cyclones or hot-gas filters. The suspensions containingchlorosilanes or dust which occur in this process are usually processedto regain chlorosilanes.

[0005] According to the methods specified in DE 36 42 285 C1, DE 37 09577 A1, DE39 41 825 A1, DE39 41 827 A1, DE41 16 925 A1, DE41 30 880 A1,DE41 30 881 A1, DE42 43 223 C1, EP0 201 200 A1, EP 0521309 A1, U.S. Pat.No. 4,690,810 they can be processed by distillation, wherein adistillation bottom product occurs containing silicon, metal chloridesand, if any, catalyst, which needs to be disposed of after renderinginert by alkali or by a treatment with water vapour. A use of themixtures of hydrolysed metal chlorides and hydrolysed chlorosilanesoccurring in such process, e.g. in metallurgical processes, is noteconomic because of the high contents of chloride and the low contentsof valuable metals, particularly silicon metal. In order to obtain avaluable product, it is therefore advantageous, to extensively separatethe solid dust compounds of the trichlorosilane-containing productmixture originating from the trichlorosilane synthesis prior to the gaswashing or the total condensation and to process them separately.

[0006] The dust occurring in the cyclones and/or the hot-gas filter canbe composed differently depending on the operating conditions in thefluidised-bed reaction, the composition of the metallurgical siliconused and the application of catalyst. It consists essentially ofnon-reacted residues of the metallurgical silicon used, chlorides likeFeCl₂ and CaCl₂, and catalyst and/or a catalyst compound, if any. Suchdust material is highly reactive and must be rendered inert prior to itsuse. Inerting in this sense means that the reactivity of the dustmaterial is reduced and its handleability is improved.

[0007] According to EP-A1-0 201 200 inerting can be achieved by mixingthe dust material with water and granulation of such mixture. Theproduct occurring is now rendered inert, but cannot be used directly asa raw material in metallurgical processes, however, because of its highchloride contents and low pH-value, combined with the easy elutabilityof metal ions, requiring a reprocessing of the product.

[0008] DE-C1-195 07 602 specifies another possibility of renderinginert, wherein the dust material is reacted in hydrous suspension withalkaline compounds, such as e.g. NaOH, CaO, Na₂CO₃, NaHCO₃ or cement,and subsequently filtered. Inerting is achieved also in this procedure,but the suspensions occurring are often difficultly filterable,resulting in filter cake with a high water and chloride content. Alsosuch products cannot be used directly as raw material in metallurgicalprocesses and must be reprocessed in expensive aftertreatment.

[0009] EP 0 416 584 A1 discloses the use of a heatable solids mixer aswell as a heatable plough blade mixer within the scope of processes ofchemical engineering.

[0010] EP 0 428 338 A2 discloses a method of rendering inert metallicsilicon containing residues that originate during the generation oforganic halogenide silane compounds. Water and an alkali compound areadded to these residues. In the process, the residues are added to anaqueous basic solution, which has been heated to 50 to 100° C., and thenfiltered.

[0011] EP 0 433 600 A2 describes a method of processing siliconcontaining residues by reaction with calcium bases or calcium carbonate.

[0012] The object of the invention is therefore to provide a method forrendering inert dust residue containing silicon metal originating fromthe trichlorosilane synthesis, providing an easy way of obtainingvaluable materials containing silicon metal, which can be used inmetallurgical processes.

[0013] This object is achieved by a method for rendering inert dustresidue containing silicon metal originating from the trichlorosilanesynthesis, characterised in that the process of rendering the residueinert is carried out in several steps. In a first step, 10 to 50 weightpercent water, in relation to the quantity of residue, and an at leastequimolar quantity of an alkaline compound, in relation to the chloridecontent of the residue, are added to the residue. Subsequently orsimultaneously, the residue is heated to a temperature of 50 to 200° C.In a further step, at least twice the quantity of water is added to thismixture, which is extensively liberated from dissolved salts byfiltration and subsequent washing with water.

[0014] It is preferred to use the alkaline compound at a molar excessratio of 2 to 10%, in relation to the chloride content of the residues.Such alkaline compounds can be, for example, NaOH, Na₂CO₃, NaHCO₃, CaO,Ca(OH)₂ or cement. Preferred are CaO, Na₂CO₃ and cement or any mixturesthereof.

[0015] In a particularly preferred modification of the inventive methodthe mixture is heated to a temperature in the range from 100 to 150° C.,preferably 120 to 140° C.

[0016] In a modification of the inventive method the mixture having beenheated to 50 to 200° C. is maintained at this temperature for a periodof 10 to 60 minutes.

[0017] When the material according to the invention is applied theproducts obtained have as a rule a chloride content of <1 weight percentin relation to the dry substance and a water content of <40 weightpercent. Such products can be used in metallurgical processes, such ase.g. the manufacture of iron alloys or the manufacture of copper,without any further aftertreatment.

[0018] The preparation of the mixture in the first step can be carriedout in a usual heatable solids mixer. Preferably mixers with in-builtmixing tools are used, e.g. plough blade mixers.

[0019] Filtration and washing is preferably carried out in a filterpress. It showed that washing the filter cake requires only smallamounts of water in order to remove the dissolved salts extensively. Thepreferred amount of water is in a range of a 1.5 to double the amount inrelation to the mass of the filter cake.

[0020] The method according to the invention will be explained in thefollowing by means of some examples.

[0021] The composition and the silicon corn spectrum of the samples usedin the experiments correspond to typical dust residue containing siliconmetal originating from the reaction of metallurgical silicon withsilicon tetrachloride and hydrogen to trichlorosilane.

[0022] The samples used had the following composition: Sample A Sample BSi 57.2 wt. percent Si 48.4 wt. percent Fe  4.8 wt. percent Fe 18.7 wt.percent Cu 26.0 wt. percent Cl 29.0 wt. percent Cl 10.2 wt. percentresidue  3.9 wt. percent residue  1.8 wt. percent (Ca etc.) (Ca etc.)

[0023] The composition of Sample A corresponds to a residue originatingfrom a trichlorosilane synthesis which was catalysed with CuCl andwherein metallurgical silicon with an Fe contents of 0.4 weight percentwas used. The composition of Sample B corresponds to a residueoriginating from a trichlorosilane synthesis wherein metallurgicalsilicon with an Fe contents of 1.9 weight percent was used.

EXAMPLE 1 (COMPARATIVE EXAMPLE)

[0024] 200 g of Sample A were mixed with 20 g cement and 1000 ml waterand heated to 75° C. Subsequently the pH-value was adjusted to 8 in thesuspension by 10 molar soda lye. After 60 min the suspension wasfiltered over a vacuum laboratory filter and washed on the filter with600 ml of water. The filter cake sucked off had a water content of 44.5weight percent.

EXAMPLE 2 (COMPARATIVE EXAMPLE)

[0025] 200 g of Sample B were mixed with 20 g cement and 1000 ml waterand heated to 75° C. Subsequently the pH-value was adjusted to 8 in thesuspension by 10 molar soda lye. After 60 min the suspension wasfiltered over a vacuum laboratory filter and washed on the filter with600 ml of water. The filter cake sucked off had a water content of 70weight percent.

EXAMPLE 3 (COMPARATIVE EXAMPLE)

[0026] 100 g of Sample B were mixed with 45.5 g Na₂CO₃ (equivalent to1.05 mol pro mol chloride), 5 g cement and 289 ml water. This adjusted atemperature of 35° C. After 45 minutes the suspension was filtered overa vacuum laboratory filter and washed on the filter with 250 ml ofwater. The filter cake sucked off had a water content of 45.1 weightpercent and a chloride content of 0.24 weight percent based on drysubstance.

EXAMPLE 4 (COMPARATIVE EXAMPLE)

[0027] 100 g of Sample A were mixed with 16 g Na₂CO₃ (equivalent to 1.05mol pro mol chloride), 5 g cement and 14 ml water. This adjusted atemperature of 52° C. After 45 minutes the mixture was mixed with 250 mlwater and filtered over a laboratory pressure filter at a filtrationpressure of 2.5 bar and subsequently washed on the pressure filter with250 ml water. The filter cake had a water content of 26 weight percentand a chloride content of 1.2 weight percent based on dry substance.

EXAMPLE 5 (COMPARATIVE EXAMPLE)

[0028] 100 g of Sample B were mixed with 45.5 g Na₂CO₃ (equivalent to1.05 mol pro mol chloride), 5 g cement and 39 ml water. This adjusted atemperature of 53° C. After 45 minutes the mixture was mixed with 250 mlwater and filtered over a laboratory pressure filter at a filtrationpressure of 2.5 bar and subsequently washed on the pressure filter with250 ml water. The filter cake had a water content of 40.3 weight percentand a chloride content of 1.4 weight percent based on dry substance.

EXAMPLE 6 (ACCORDING TO THE INVENTION)

[0029] 100 g of Sample A were mixed with 16 g Na₂CO₃ (equivalent to 1.05mol pro mol chloride) and 14 ml water and heated to 130° C. After 45minutes the mixture was mixed with 250 ml water and filtered over alaboratory pressure filter at a filtration pressure of 2.5 bar andsubsequently washed on the pressure filter with 250 ml water. The filtercake had a water content of 23.4 weight percent and a chloride contentof 0.27 weight percent based on dry substance.

EXAMPLE 7 (ACCORDING TO THE INVENTION)

[0030] 100 g of Sample A were mixed with 16 g Na₂CO₃ (equivalent to 1.65mol pro mol chloride), 5 g cement and 14 ml water and heated to 130° C.After 45 minutes the mixture was mixed with 250 ml water and filteredover a laboratory pressure filter at a filtration pressure of 2.5 barand subsequently washed on the pressure filter with 250 ml water. Thefilter cake had a water content of 25.5 weight percent and a chloridecontent of 0.7 weight percent based on dry substance.

EXAMPLE 8 (ACCORDING TO THE INVENTION)

[0031] 100 g of Sample B were mixed with 45.5 g Na₂CO₃ (equivalent to1.05 mol pro mol chloride) and 39 ml water and heated to 130° C. After45 minutes the mixture was mixed with 250 ml water and filtered over alaboratory pressure filter at a filtration pressure of 2.5 bar andsubsequently washed on the pressure filter with 250 ml water. The filtercake had a water content of 32 weight percent and a chloride content of0.24 weight percent based on dry substance.

EXAMPLE 9 (ACCORDING TO THE INVENTION)

[0032] 100 g of Sample B were mixed with 45.5 g Na₂CO₃ (equivalent to1.05 mol pro mol chloride), 5 g cement and 39 ml water and heated to130° C. After 45 minutes the mixture was mixed with 250 ml water andfiltered over a laboratory pressure filter at a filtration pressure of2.5 bar and subsequently washed on the pressure filter with 250 mlwater. The filter cake had a water content of 37 weight percent and achloride content of 0.38 weight percent based on dry substance.

[0033] As can be seen from the examples, the method according to theinvention provides products with water contents below 40 weight percentand chloride contents below 1 weight percent (in relation to the drysubstance). Therefore these products are particularly suitable for autilisation in metallurgical processes.

1. A method for rendering inert dust residue containing silicon metaloriginating from the trichlorosilane synthesis, characterised by thefollowing steps: the residues are mixed with 10 to 50 weight percentwater, in relation to the quantity of residue, and an at least equimolarquantity of an alkaline compound, in relation to the chloride content ofthe residue; the mixture is heated to a temperature ranging from 50 to200° C.; the heated mixture is mixed with at least the double amount ofwater; and subsequently the mixture is extensively liberated fromdissolved salts by filtration and subsequent washing with water.
 2. Amethod according to claim 1, characterised in that the alkaline compoundis provided at a molar excess of 2 to 10%, in relation to the chloridecontent of the residues.
 3. A method according to claim 1 or 2,characterised in that the alkaline compounds used are NaOH, Na₂CO₃,NaHCO₃, CaO, Ca(OH)₂ or cement.
 4. A method according to claim 1 or 2,characterised in that the alkaline compounds used are CaO, Na₂CO₃ andcement or mixtures thereof.
 5. A method according to any one of claims 1to 4, characterised in that in the first step the mixture is heated to atemperature ranging from 100 to 150° C., preferably 120 to 140° C.
 6. Amethod according to any one of claims 1 to 5, characterised in thatafter heating the mixture is maintained at this temperature for a periodof 10 to 60 minutes.
 7. A method for the execution of a metallurgicalprocess, in particular for the production of an iron alloy or copper,comprising the following steps: a) rendering inert dust residuecontaining silicon metal originating from the trichlorosilane synthesis,comprising the following steps of rendering inert: the residues aremixed with 10 to 50 weight percent water, in relation to the quantity ofresidue, and an at least equimolar quantity of an alkaline compound, inrelation to the chloride content of the residue; the mixture is heatedto a temperature ranging from 50 to 200° C.; the heated mixture is mixedwith at least the double amount of water; and subsequently the mixtureis extensively liberated from dissolved salts by filtration andsubsequent washing with water; b) utilisation of these inert residues inthe subsequent metallurgical process.